[chaos mode] Choose a random e-node from the e-class rather than the best one

This is what `wasm-mutate` does to generate pseudo-random semantics-preserving Wasm programs from an input Wasm module. We could also do it in our own mid end to exercise the back half of the compiler pipeline.

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I was talking with @jacarte the other day as well and he had a similar idea: do the `wasm-mutate` e-graphs thing (select a random e-node from the e-class rather than the best e-node). I guess this would also fit in with the chaos mode work.

_Originally posted by @fitzgen in https://github.com/bytecodealliance/wasmtime/issues/5967#issuecomment-1464716718_